1. Field of the Invention
The present invention relates to a hard amorphous thin film coating (sometimes referred to as simply "coating" or "thin film" hereinafter) having lubricity, to an iron-base metallic material having on the surface thereof a hard coating of low friction, and to a process for producing the same. More specifically, it relates to a hard thin film coating of amorphous carbon-hydrogen-silicon, which is hard and particularly low in friction coefficient. It also relates to an iron-base metallic material having on the surface thereof a lubricant amorphous carbon-hydrogen-silicon, and to a process for producing the same.
2. Description of the Related Art
Forming hard coatings of metal carbides and metal nitrides such as vanadium carbide (VC), titanium carbide (TiC), and titanium nitride (TiN) on the surface of iron base materials used as tools and metal molds by salt bath immersion method, chemical vapor deposition(CVD), physical vapor deposition (PVD) and the like, is well known. It is practiced in the art of forming thin films and surface-treating iron base materials with the purpose to improve resistances against abrasion and seizure. Those coatings generally are hard and have a Vicker's hardness, Hv, of from 2,000 to 3,000, however, the coatings themselves are not lubricant and the friction coefficient thereof is not so low (from about 0.2 to 0.8, in friction against steel, without applying a lubricant). Accordingly, the sliding resistance in the presence of a counter material increases and causes wear loss to occur on the surface of the coating on one hand, while induces damage on the surface of the counter material on the other. This has therefore constituted a problem yet to be solved.
Concerning lubricant thin film coatings, there is also known thin films of amorphous carbon produced by a process such as plasma-assisted decomposition of hydrocarbon gases. This type of thin film possesses a friction coefficient as low as 0.01, and therefore its practical application to sliding members appears promising. Such favorable amorphous carbon films are, however, structurally unstable, and their tribological properties are subject to atmospheric conditions under which the films are being used. More specifically, it has been a problem in such films that they easily increase their friction coefficient to a high 0.2 in case of using them in air.
There is proposed, accordingly, a measure to solve the aforementioned problems, which comprises incorporating additional metals to carbon coatings to thereby stably obtain a friction coefficient of 0.1 or lower against steel in ambient atmosphere (U.S. Pat. No. 4,525,417). It is reported that a low friction coefficient of 0.07 is obtained in carbon films containing from 5 to 20% by atomic of silicon, however, a value as low as 0.05 or less, e.g., 0.03 and 0.04, has not yet achieved.
A friction coefficient of 0.1 or lower is indeed a low value if compared as an absolute value, but considering reducing the value from 0.07 to 0.03, it can be easily realized that the coefficient is halved, and therefore the reacting force incorporated therein is also reduced to a half. It is therefore, a great matter concerning the practical application of lubricant films. In the aforementioned process, anyhow, it was not possible to obtain a film having a friction coefficient as low as 0.05 or less.
It has been recently reported a method of forming a diamond-like hard carbon coating having an Hv as high as 10000. This film is therefore excellent if only hardness is to be concerned. On the other hand, the film is so hard that it requires a very smooth surface, since any irregularity on the surface can easily damage the counter material, and this in turn has become a problem. It is not possible in the state of art to form a smooth and uniform film, nor is easily obtained a smooth surface by post treatment of the film such as polishing. Accordingly, practical application of those films to sliding members has been a problem of those films. It is also a problem of those films that they can achieve a friction coefficient of 0.1 even on smooth surfaces at best, which is a value one order higher as compared with those of amorphous carbon films.
It is further reported a process for forming amorphous silicon carbide films. Such films have a hardness Hv of about 2,000 and a low friction coefficient of about 0.1, but it is also reported that those properties are easily impaired with deviation in the composition of the film from a stoichiometric value (See Thin Solid Films, Vol. 139(1986), p.275, and Japanese Patent Publication (unexamined) No. 184681/1985).
There further is known a method which comprises applying solid lubricants such as molybdenum sulfide (MoS.sub.2) and graphite to thereby improve lubricating properties. Such method may yield a low friction coefficient of 0.05 or lower depending on conditions, however, it also had a problem concerning its durability, since the coated layer is apt to fall off or to be abraded in its use for a long period of time.
Furthermore, aforementioned carbon-based films and silicon carbide films suffered low adhesibility with iron-based materials.